Analyzing apparatus



g- 22, 1950 w. H. WANNAMAKER, JR 2,519,

ANALYZING APPARATUS Original Filed March 27. 1945 3 Sheets-Shet 1 FIG. I

JNVENTOR. WILLIAM H.WANNAMAKER JR ATTORNEY.

Aug. 22, 1950 OUTPUT CURRENT (MICROAMPERES) FINAL VALUE OF VARIABLE Original Filed March 27, 1945 3 Sheets-Sheet 2 FIG. 3 FIG. 5 200 I00.

(9 E D 4 l-IJ 1 loo 50 U] Q E O 0 DJ E 0 -2 -3 -5 -I0 VOLTS INPUT (0c) INPUT VOLTS FIG 4 LAG FOR RATE OF oNE PER SECOND I P I CURRENT INTO PRocEss u I l 5 REFERENCE (1) S PEN MOVEMENT l I l 50 TIME I IS SECONDS I50 FIG.6

I RANGE FACTOR=4.0

lG I 46 T 47 4a 47 4a o.5v L I l6 THE INVENTOR.

WILLIAM H. WAN NAMAKER JR- ATTORNEY.

Aug. 22, 1950 w. H. WANNAMAKER, JR 2,519,615

ANALYZING APPARATUS W N N W N MS IS 18 S 1.8 5 T MT MT MT MT L L L L L m m H R0 R0 R0 m o/owv M Oopv %Pv m S 2 2 2 8 5 l r 53 4M 4. 5. t e E E E E m mm 0. or or or S N NA NA A A N A A R 3 B AR M M B T T T T T W Ma ma wx wflm S R E M R E M R E M R E M R E P P R S P R S P R S P R S P R S Original Filed March 27, 1945 INVENTOR. F.G. WILLIAM H.WANNAMAKER JR.

ATTORNEY Patented Aug. 22, 1950 2,519,615 ANALYZING APPARATUS William H. Wannamaker, Jr., Flourtown, Pa., assigner, by mesne assignments, to Minneapolis- Honeywell Regulator Company, Minneapolis, Minn, a corporation of Delaware Original application March 27, 1945, Serial No. 585,125. Divided and this application August 19, 1947, Serial No. 769,555

11 Claims. (01. 23561) The present invention relates to apparatus for analyzing and studying automatic regulation and control procedures, and, more especially, pertains to apparatus for artificially creating or duplicating characteristics of various physical processes in order to facilitate investigation of the effect of said characteristics and/or the effect of inherent qualities of the automatic control apparatus on the automatic regulation or control obtained.

In fundamental investigations of automatic control procedures, a need often arises for process simulation or duplication. Automatic control procedures may be studied in the field on actual control applications, but such investigations are ordinarily very difiicult to carry on because the process under investigation is subject to changing conditions over which there is little or no control. Moreover, in actual control applications in the field, the introduction of intentional load changes, disturbances orcontrol point shifts, which it is necessary to make in order to determine the dynamic action of the control system, is generally not permissible. As a consequence, various methods and apparatus have heretofore been proposed for the purpose of facilitating the investigation of automatic and apparatus.

The apparatus of the present invention oiiers advantages of versatility which render it especially suitable and desirable for accomplishing such investigations in the laboratory. Specifically, the dynamic action of the control system applied to control or regulate the simulated process may be readily and accurately observed, and any factor, either in the simulated process or in the automatic controller, may be easily simulated and altered.

An object of the invention is to provide imcontrol procedures proved apparatus for artificially creating or duplicating characteristics of physical processes. It is a more specific object to attain this result by means of an electrical analogue utilizin elec trical resistance and capacitance circuits.

Another object of the invention is to provide in conjunction with indicating type automatic control apparatus improved apparatus for studying the effect on the indicating means, and thereby on the characteristic under investigation, of variations in such factors as throttling range and/ or reset rate.

A further object of the invention is to provide improved apparatus for studying the effect on the indicating means, and thereby on the characteristic being investigated, of such changes in a process, or in various different processes, as

shifting of the control point, load changes, control agent supply changes, and/or changes in socalled dead time. Dead time may be defined as the elapsed time between the occurrence of a change in a factor operative to ultimately cause a change in the measured characteristic and the time when such change in the measured characteristic actually takes place.

Another specific object of the invention is to provide a simple and efiicient electrical circuit arrangement for simulating the flow of a condition controlling agent such, for example, as the supply of heat to a furnace or the supply of liquid to a tank. It is also an object of the invention to provide such an electrical circuit arrangement in which the voltage in any portion of the electrical network through which the current flows represents the magnitude of the condition, such, for example, as temperature or liquid level, being simulated.

A still further specific object of the invention is to provide a simple and efiicient electrical circuit arrangement for creating through an impedance load a current flow which is practically independent of the magnitude of said impedance. It is also an object of the invention to provide such an electrical circuit arrangement including readily adjustable means for varying the amplitude of said electrical current and also the range through which said current may be varied.

A still further object of the invention is to provide improved apparatus for investigating the effect on the indicating means, and thereby on the characteristic under investigation, of changes in the controller dead zone, changes in the characteristic of the control valve employed, and also changes in any factor involving any operation of the complete system under investigation. It is a specific object of the invention also to provide a simple and efficient electrical circuit arrangement for artificially simulating characteristics of a control valve and for readily adjusting the size and range of the control valve.

It is a further object of the invention to pro- Vide improved apparatus for ascertaining the operating characteristics and relative merits of different types of automatic controllers applied to similar processes.

This application is a division of my prior a plicatlon, Serial No. 585,125, filed March 27, 1945, which issued as Patent No. 2,453,653 on l lovemher 2, 1948, and in which the electrical circuit arrangement for introducing dead time of a, pre-determined duration in the transfer oi electrical current from one point to another point in an electrical network is claimed.

The various features of novelty which characterize my invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, however, its advantages, andspecific objects attained by its use,

referencesh'ould be had to the accompanying drawings and descriptive matter in which I haveillustrated and described a preferred embodiment of the invention.

Of the drawings:

Fig. 1 is a schematic illustration of a srererr aform of apparatus according'to the-presentin vention;

Fig. 2 shows in ner the details of matically in Fig. 1;

more or less diagrammatic manthe various 'unitsshown selfe Fig. 3 is a graph showing the output current versus applied input voltage relation of theou'r rent input unit of Fig. 2;'

Fig. e is a graph illustrating the effect --of the dead time unit of Figs. -1 and 2;

v-Fig. -5' illustrates calibration; curves with-thevacuumtube voltmeter of Figs. 1 and 2';

Fig: 6- isagietph showing a reactionpurve-oi a process simulated'by means of the arrangement of Figsnl-and 2; and

Fig. 7 is a graph showing a number'of recover curves .ror the process having the reaction curve of Fig."6L

Prior workers the'art have proposec'tecnniqeesor applyingcontrol to simulated pro ess'e's;

The techniques proposed, however, tiiere rim-nee in theirepplication'in that they did not lendthemselves readily to adjustmentfor duplicat alga variety of multiple capacity processes.

sistances and condensers, which alterations may 1 obtainable be easily accomplished in practice, as by means of plug and jack'connections.

Inasmuch -as a direct relationship has b'een foundto exist-between electrical units-and thermal and hydraulic units, the physical con sta'nts' of the resistances and condensers utilized may be'calculated using well known theory.- The relationship between electrical, thermal and h draulic-units -iS illilStIamtBdby means o'r the following table. In the table the unitsai'ebased on point values inorder to avoid the complexity of power functions. The time basis of the analogy may cap" bear'any chosen convenient ratio us that of the actual process'although entered time relation may introduce "complications when the control system involves integral and derive."- tive' functions.

Most industrial controllers are provided with some form of pneumatic unit, electrical contact means, or power-set slider on a resistance slidewire for accomplishing the desired control actions on the process to which the controller is apfpl ie' rl; The apparatus of the present invention is readily adaptable for use with any one of such industrial controllers, but for purposes of illustration I have shown in Fig. 1 the controller as eo'in'p'risi'rig--"a self balancing potentiometer I equipped with pneumatic control means supplied with air from a suitable source through a pipe 2 for causing the air pressure to a diaphragm valve motor =3=to position a sliding contact t along a slidewire resistance 5. The potentiometer l andnle pneumatic control means including the 'd'iaphrjagm'mb'tor 3 inay be of the type commercially manufactured and sold by The Brown Imminent Company and respectively disclosed in the Wills application Serial Number 421,173, filed December 1:, 1941 ,"now Patent- No. 2,423,540

of'July"8,' TMV-yand in the Moore-Patent 2,125,08i,"

unit '9 'whichl havefor convenience termed a dead-timeunit: The output terminals ie of the dead time" unit-9 areconnected to the 'n'put terminals-H of a uhit l2, conveniently'ter current input unit, having output 'terin.nals 3' which areconnected to the-input I terminals t l of 'a'unit cesigned to simul ate'the capacity a transfer lags or a p ocess "under investigation" and-comprising a res'istafnce capacitance net work It.

The} values the network 1-5 tfil 6f Fig; 1 the simulated process under controh The various flvalues'fof the condensers andresistances are "so 'cho'senas to i'r'itrbduceadelay in the volt age transfer through the-network firomthe' input terminals 14 to the output terminalsfi e corre spending "t the'f-d'e'lay "(rescuing nom- "the and transfenlags of the process) "occur;- ring between the time a chang'e' in the supply of a controlling gent; to an actual process under ade nd the time when the process A 'astab'iliz'ed condition 'withthe new supplyof controlling'agent';

The outputterminals 1a or the network ttare connected "to 'th iiiplittf'ilii'nal's H of f l'ec tronic voltmeter l8 havingoutput ter Is 19 which are connected to iiipilt terminals of the potentiometer petitioner l As be Seen by re erence to Fig. 2', which detail the various units of 7 illustrates in more tude which is impressed on the terminals 8 of a"- or resistanceand capacitance in determine the" characteristics 10f j ber of condensers and the number of points oneach section of the multiple contact switch may be larger or smaller as desired. A physical embodiment of the arrangement shown actually comprised twenty-five condensers and twentyfive points on each section of the multiple contact switch The ratchet type multiple contact switch 2| may be of any suitable type and, for

example, may be of the wellknown telephone type.

' An electronic pulse circuit, indicated generally acre, and an associated relay 25 are provided for actuating the multiple contact switch 2| to advance the contact blades 22 and 23, one step at a time, from one point to the next succeeding point.

One input terminal 8 of the dead time unit is connected to a common terminal of eachof the condensers '20, and the other input terminalfi is connected to the rotatable contact arm 22. The other terminal of each of the condensers 2B is connected to an individual one of the points on the contact switch section associated with, the r Consequently, asfthe" rotatable switch arm 22.

5 od is preferred because variation in the dead time is easier of accomplishment by this method.

The adjustable electronic pulse circuit 24 is supplied with energizing current from a source of alternating current of commercial frequency 19; which isadapted to be connected to the termina1s 26. The circuit 24 includes two electronic tubes designated by the numerals 21 and 28 which, asshown, are heater type triodes. Energizing current is supplied to the filaments of is each of the triodes 21 and 28 from the secondary winding of atransformer 29 having its primary winding connected across the terminals 28. A potentiometer resistance 38, provided with a sliding contact 3|, is also connected across the ter- ,minals26, the cathode of the triode 27 being connected to the slidingcontact 3!. The control grid of ;thetriode 21 is connected through a suitable resistance to the lower terminal of resistance 30, as seen in the drawing. Accordingly,

5 the voltage drop; appearing between the contact.,3l;and,the ;lower terminal of resistance 38 providessuitable bias voltage for the control grid oLthe triOdfizJZ'L; The magnitude of this bias voltage .may ;:be adjusted as desired by sliding the contact 3i along .thelength of resistance 30.

The anode. ofthe triode 21 is connected by a condensfirsflto the upper terminal of the resistance. ;30'. .-'I hus, alternating voltage of commericalfrequency is supplied to the anode cirarini22 is advanced, step by step, from driecon ias cuit rof-the triode21, .whereby the triode 27 is tact point to the next, an electrical charge applied to each of the condensers/20 in succes} sion 'from the slidewire resistor 5. The magnitude of the charge applied to the condensersis directly determined by the position of the con ance 5.

As shown in Fig. 2, each of thesecond men-f tioned terminals of the condensers 20 is also con periodically rendered conductive at the frequencyofthe supply source.

Theoutput circuit of the triode 28 is connected to the terminals 26 in such manner that this tri- 140 ode is adapted to be rendered conductive during tact 4 along thelength of the slidewireresist -t half, cycles which alternate with those during which the triode-2'! is adapted to be rendered conductive. The output circuit of triode 28 includes: the;operatin coil 33 of a relay 34 which nected to an individual one of he Contact p is adaptedito .close a switch 35 when the relay is on the contact switch section which is associated with the rotatable contact arm 23. The switch arm 23 is connected to one output terminal ID of the dead time unit and thereby to one input terminal I i of the current input unit l2, and the other input terminal ll of the latter is connected to each of the first mentioned terminals of the condensers 28. The two switch contact arms 22 i and 23 are out of step by one position and the wiring is such that each previously charged condenser applies its potential to the input terminals ii of the current input unit 12 for a period equal to the time between steps. Specifically, the switch arm 23 is shown as being ahead of the switch arm 22 by one step so that the dead time unit 9 effectively produces a dead time equal to eleven times the stepping or pulsing period in the arrangement shown. If desired, the switch arm 22 may be made to move in step withswitch arm 23 and the wiring from the condensers-20 so! that the switch arm 23 will beelectrically ahead of the arm 22 by one step. When a larger number of condensers is utilized, the stepping or pulsing period remaining the same, the dead time is correspondingly larger. For example, when twenty-five condensers are employed, the dead time is equal to twenty-four times the pulsing period. It is noted that the dead time may be increased or decreased in duration by either 5 coil 33 for 0 its associated cathode.

energized. The switch 35 is opened when the relay is d'eenergized, being biased by gravity or other suitable means to its open position. A condenser 36 is connected in shunt to the operating smoothing out the current flow through the latter and, thereby, for eliminating relay chatter. The input circuit of the triode 28 is controlled in accordance with the voltage produced across the condenser 32 which is included in the'output'circuit of the triode 21.

When the operation of the apparatus is initiated, the potential drop across the condenser 32"is zero, and consequently, the control grid of the triode 28 is then at the same potential as The triode 28 is then fully-conductive and energizes th relay 34 to close. the :switch'35. This action serves to close an: energizing circuit to the operating coil 3'! of the relay 25 from a battery 38. As a result, the

5 rrelay '25 theniadv'ances the contact-arms 22 and to the contact points of the switch 2| arranged 23 101' the'switching mechanism 2| to the next position? 'As the triode' -fl is periodically rendered conductive during alternate half cycles of the commercial alternating current supply source, a

ltidncontinu'es until the conduction of the triode asaogeia' 28 is reduced to such an'extenttha'ttherelayfl is deenergized switch 35 is actuated to its openposition whereupon relay 25 is deenergized. Relay 25, as shown,

is also provided with a switch 39 which, when closed, short circuits the terminals-of the condenser 32 and consequently discharges the latter. Switch 39 is actuated by gravity or other suitable means to its closed position when relay- 25 is deenergized and is moved into :its open-position when relay 25 is energized. Accordingly," when 'triode 28 is rendered non-conductive due to the charge stored by conden'ser-32,*the-deenergizing action of relay 25 operates to permit switch 39 to close and thereby short-circuit condenser 32. This discharges condenser =32,--and hence, permits triode-28 to again conduct and energize relay 34. Such energization of relay 34- operates to cause-energizationof relay '25, which ing contact 3| along-the length of theresistance 30. For example, by increasing thenegative bias applied tothe controlgrid of the triode -21, the time required tocharge condenserSZ sufllciently negative to rendertheitriode 28 non-conductive is correspondingly increased, and vice versa.

In accordance with the present-invention, the sliding contact 31 is adjusted toa position-along the length of the resistance 30 corresponding to the magnitude of the dead .timeit is desired to 1 introduce into .the simulated process. In this manner, a readily adjustable means is provided for varying the time interval between theapplication of a change in the voltage-applied to theinput terminals of the dead time unit and the time when that voltage-change appears between the output terniinals'of the dead time unit. This delay in the appearance of the voltage change between the input and outputterminals of the dead time unit corresponds to the delay occurring in actual complete control systems between the time of application of a change at some point in'the system, which is operative. to

influence a factor of the process under control, and the time when such change has reacted through the complete system to cause a change in the opposite'sense at the point :where the original change was initiated.

The current input unit 12 is :employed'to provide a source .of current in its output circuit which is regulated in value by. adjustment of the voltage appliedto its input terminals and-.the.

range of values'of which mayxalsobezvariedas desired. A characteristic of the currentsinput unit is that it may be utilized=rto simulate a line-n60 chanical valve, for example, regulating' thexflow:

Upon such occurrence, theofafuel to a -furnace,--the azalve opening-slicing ads. 2

justed in accordance-with the voltag zimpressedon the'input terminals H. Means to beedescribed are also included in ttheicurrent einpnt '2 unit for varying the sizeof' thez-simulated-valve.

Specifically, the current input unit [2 impresvided with twoheater type triodes-respectively indicated by the reference numeralsAIl-anddLea batteryAZ for supplying anodeorbutnut-cumentto both triodes in series, aibattery fl-forsupp1ying suitable bias -voltage-:to the input supplying. suitable bias voltage. to: the? input :cir,

circuitof the triode- 4 I and anadjustable resistance Mater suitable regulated voltage supplies energized from a commercial alternating current source may-beemployed, instead of batteries 42 and 43, if desired. A condenser 45, which preferablyis of small :value, is connected across the input terminals H for smoothing out the voltage pulsa tions applied to the input terminals of the current input unit 12 from the output terminals of the dead time unit 9 andto stabilize -the poten-- grid of triode 40 during the Tracing through the complete circuit having connected therein-the impedancenetwork [-5 and the anode-cathode circuits of the triodes 40 and 4|, Equation'l may be obtained by application of Kirchhofis' principle that the sum of the voltage drops in a-closed 'eircuitmust be'equal-to zero.

whereiRls is the load resistance presented by the impedance network [5 between the output terminals l3 of thecurrent input unit 12; I1) isthe output current flow from the current input-unit l2ithrough the impedance network I5; R44 is the resistance of the cathode bias resistor ,44;u4u and 41 are the amplification factors of the-triodes 40 a d 1 re$pe0tively;.R4o and R41. arethe anode to cathode resistancesof tubes 4.0 and 4|,'res pe ctively; fEn is the signal voltage impressed across the'input terminals ll of the current input unit l2 frolnthefilidewire resistance 5; E43 is the .Voltage of batterydS; and]: is a constant.

Differentiating Equation 1 with respect to R15, Equations 2 and 3 are obtained:

Equation 4 is obtained by transposing-,and by substituting, P. for

44 40 41 R15 in Equation 3.

As those skilled in theart will;unde,rstand,- -'rthequantity represents the ratio of the current change in the output circuit of the current input unit I2 for a given change in impedance between the output terminals I3. By inspection of Equation 4 it may be observed that the current In flowing in the output circuit of the current input unit I2 is substantially independent of the magnitude of the load impedance R15 over a range extending from zero up to one megohm and higher values of resistance.

Merely by way of illustration, and not limitation, it is noted that, when triodes 40 and 4! are of the commercially available 7F! type, the other circuit components of the current input unit I2 may have the following values:

Value 5,000l50,000 ohms. 300 volts. 80 volts. 0.05 mictofarad.

When these circuit components are employed and resistances R15 and R44, for example, are assigned values of 500,000 ohms and 100,000 ohms respectively, the value of the quantity 40 4l 40 41)" 4lk may be readily calculated and found to be approximately 0.001 when the anode to cathode resistances of each of the triodes 40 and 4| is assumed to be 20,000 ohms and the amplification factor it of each of the triodes is assumed to be 70. Accordingly, the change in output current flow from the current input unit I2 upon change in the magnitude of the load resistance R15 will be or, in other words, only 0.1% of the ratio Thus, if the change in load resistance R15 is 250,000 ohms, the change in output current flow from the current input unit I2 With respect to its original value will have the value of or 0.05%. Consequently, there is an insignificant change in the flow of current from the output circuit of the current input unit I2 upon change in the value of the load resistance R15 over an extended range.

Fig. 3 is a graph illustrating curves of anode or output current flow in the output circuits of the triodes 40 and 4E plotted against voltage impressed on the input terminals II from the dead time unit 9 for various values of resistance 4-4. In this graph it is contemplated that the load inipedance connected in the output circuit of the triodes 40 and il may have any value from zero to at least one megohm and to higher values, if desired.

The output current flow derived from the current input unit i2 is impressed upon the input terminals I4 of the resistance-capacitance net- Work 55 constituting the simulated process. The value of the current fiow to the resistance-capacitance network I is a function of and is determined by the voltage impressed on the input terminals II of the current input unit I2 from the must be taken into account.

dead time unit 9. The output voltage variations which appear between the output terminals I6 of the resistance-capacitance network I5 simulate the variable being measured for an actual process, such, for example, as the temperature within a furnace. In order to measure the voltage variations between the terminals IS, the vacuum tube voltmeter I8 is employed in conjunction with the potentiometer controller I. The vacuum tube voltmeter I8 serves as an impedance matching device to permit the proper functioning of the potentiometer controller I, which controllers ordinarily are designed to work from low voltage, low impedance circuits.

The resistance-capacitance network I5 includes a number of resistances 46 and also a number of parallel connected resistances 41 and con densers 48. The condensers 48 may be of any suitable type and, for example, may be paper type condensers or may be dry electrolytic condensers. In the aforementioned practical working embodiment of the invention, the condensers 48 are wired to colored jacks on a Bakelite panel, and about 30,000 microfarads of capacity are used together with about thirty adjustable resistors 46 and 41. These are adapted to be connected by means of jumpers into jacks to form various types of mesh circuits. A built-in bridge circuit and null indicator (both not shown) are also included for circuit measuring purposes. It is noted that when the condensers 48 are of the electrolytic type, a circuit calling for higher shunt resistances than are inherent in the condensers, in the form of leakage, may not be utilized. In addition, variation of capacity and leakage, both with ambient temperature and impressed voltage, While electrolytic condensers impose such limitations upon the use of the apparatus, a real advantage is gained in their use, however, in that many industrial process applications may be simulated satisfactorily with considerable saving in cost and space.

The vacuum tube voltmeter .l8 comprises a sharp cut-off heater type tube, preferably a pentode but shown for convenience as a triode 49, which is operated at a low plate voltage. A large series grid resistor 50, connected between the control grid of triode 49 and one output terminal I6 of the resistance-capacitance network I 5, is utilized to reduce grid current to a low value. Anode current is supplied to the triode 49 from a battery 5I through an adjustable resistance 52 and a fixed resistance 54. The potential drop across an adjustable portion of the load resistance 52 provides the voltage to operate the potentiometer controller I. In order to make the potentiometer controller read upscale for an increasingly negative voltage applied to the control grid of the triode 49, an adjustable resistance 53 is provided in shunt to the battery 5| in series with a resistance 55 to derive a voltage which may be opposed to the voltage drop across resistance 52. That is to say, the voltage drops across the resistors 52 and 53 are equal, whereby their resultant is zero, when zero voltage is impressed on the control grid of the triode 49 from resistancecapacitance network I5.

Preferably, the anode voltage supply derived from the battery 5| and the source of voltage for energizing the heater filament of triode 49 are regulated. Such regulating means are desirable especially when these voltages are derived from an alternating current source. Desirably, two vacuum tube voltmeters may be provided so that one may be employed to actuate the po- 11 tentiometer controller, as shown, and the other may be employed to actuate a recorder to show the demand or the actual fuel flowing, or may be utilized to indicate the voltage within any portion of the process mesh circuit 15.

In the aforementioned practical working embodiment of the present invention, an impedance measuring bridge is built into the panel board, with jacks for inserting leads from the various circuit components, and an ordinary electronic amplifier and electron-ray tube of the indicating type are utilized as a null indicator.

' In the operation of the arrangement shown in Fig. 2, the diaphragm motor positions the sliding contact 4 along thelength of the slidewire resistance 5 in accordance with the variations in the applied air pressure, and thereby, according to the variationsin magnitude of the voltage impressed on the input terminals of the potentiometer controller 8 Since the number of valve provided on the slidewire resistance 5, this numher is made suitablyhigh and, for example, may be 200, although more convolutions or fewer convolutions may be employed. A greater number of a valve positioner. The sensitivity obtained with 200 convolutions has proved satisfactory when used in conjunction with the dead time unit 9 with its step function, as described.

It is noted that in low capacity processes where low rates of switching would be objectionable, dead time to any large degree is. generally not met in actual practice and the consequent fast pulsing of the dead time unit 9 provides a satiscondensers 20 used with the switch 2 i may stand idle for the necessary periods without appreciable loss of charge, while the current drain during the period each is connected. to the current input unit i2 is small,

The reason for utilizing a current input unit [2' of the type disclosed-and described willnow be explained. As those skilled in theart understand, most processes are non-regenerative. For example, a burning fuel releases a certainnumber of heat unitsregardless of the temperature within the fire box. If the supply of fuel is reduced, heat does not flow back into the fuel sup ply as fuel. It is for the purpose of simulating such a burner that the current input unit l2, consisting of a high gain amplifier having considerable negative feedback, is utilized. With this current input unit 12, a current will flow in the output circuit at a predetermined value practically independent of the impedance into which it is working when a fixed. potential is applied to its input terminals H. When the load into which the current input unit !2 is working is composed of condensers and resistances,-as in the'mesh analogue circuitl5, this flow of current simulates flow of heat, while the voltage in any portion of the resistance-capacitance mesh represents temperature. If the demand goes to zero, the current input to the terminals id of the resistance-capacitance network It from the current inputunit l2 will be made zero, the current .20 positions is limited to the number of convolutions v2 of convolutions than 200 may require the use' will not'flow back into the source, the current system has been designed to approximate a semilogarithmic valve characteristic in its action.

The characteristic of the valve may be predetermined as desired by suitable design of the slide-wire resistance 5, as for example, by winding the resistance on a formhaving a shape corresponding to the valve characteristic desired.

Figs. 3, 4 and 5 illustrate, in more or less dia-, grammatic manner, typical performance characteristics of the apparatus disclosed in Figs. 1 and 2. Fig. 3, previously referred to, illustrates the current flow into the resistance-capacitance network IE or process analogue for diiferent values of impressed voltage on the input terminals of the current input unit i2. It is noted that the current value is practically independent of the dynamic impedance of the process mesh circuit I5 and that the equivalent valve size may be chosen as desired by changing the self-biasing resistor M.

Fig. 4 illustrates the manner in which the dead time unit 9 delays the voltage from the valve slidewire, while Fig. 5 illustrates severaldifferent calibration curves obtainable with the voltmeter 18 by means of which the equivalent of suppressed range studies, for example, may be made.

Although it is possible, by means of the method and apparatus shown in the drawings and herein described, to approximate a particular process through regulation of its resistance and capacity, it is only necessary in practice to arrange certain combinations of capacity, transfer lag and dead time to represent whatever particular characteristics are desired. In-this manner, the ef fect of various characteristics in automatic control may be investigated by means of the apparatus described without actually attempting to set up specific processes. A principal application for the arrangement'disclosed, therefore, is to simulate process characteristics rather than to duplicate a specific physical process, although the latter may also be done if so desired. Examples of types of problems which may be solved in accordance with the present invention are given hereinafter.

One typical use to which the invention may be put is the determination of the reaction curve of a process. A reaction curve, an example of which is shown in Fig. 6, shows the dynamic action of the process to a sudden change of control agent input and is obtained by fixing the position of the control valve and allowing the measured variable to line out at a Value slightly below the control point. During this period the process is under manual control rather than automatic control.

In obtaining a reaction curve of a process, for example that shown in Fig. 6, a small sudden movement of the valve to a new fixed position is made. If the process under investigation has self-regulation, the measured variable will gradually rise to a new balanced value. If the mag-- nitude of the change has been properly selected, the final value should be slightly above the control point. The resulting change with time of the measured variable is the process reaction curve. It is noted that this reaction curve includes the effect of measuring and controller lags as well as the process characteristics.

Another use to which the invention may be put is the study of the relative merits of suppressed range controllers. Controllers with a suppressed range scale have come increasingly into use for the purposes of more accurate readability and control of the magnitude of the controlled variable. For example, if it is desired to control a temperature of 250 F. it is becoming common practice to use a controller calibrated 150 to 300 F. instead of to 300 F. This suppressed range has an effect on the controller adjustments because of the different rate of controller pen motion for the same rate of temperature change.

In conducting an investigation of the effect of suppressed range controller scales upon optimum controller adjustments, the first step is to select a suitable process which approximates an actual process of the type upon which controllers pro-- vided with controller adjustments are employed. Such a controller might, for example, be a proportional reset controller. This type of process generally involves slight transfer lag with moderate process capacity. For the purposes of this investigation, dead time is not required and is, accordingly, omitted. To this end, a switch 54 is provided for directly connecting the sliding contact 4 to the control grid of the triode 40 and for shunting out the dead time unit 9.

A representative process, which may be employed for investigating the effect of a suppressed range controller scale upon controller adjustment, is illustrated in the graph shown in Fig. 6 which represents the reaction curve of a multiple capacity process simulated by means of three resistance-capacitance networks, as shown in Fig. 6. The values of resistance 4? and condenser 48 in each network except the first, namely that connected to the input terminals [4, are respectively 300,000 ohms and 300 microfarads. The values of the resistance 4! and condenser 48 connected across the input terminals M are respectively 120,000 ohms and 750 microfarads. In each case the resistance 4! includes the leakage resistance of its associated condenser. Each resistance-capacitance network is connected by a resistance 46 of a value of 40,000

ohms to cause an equivalent temperature drop between the condensers. In order to obtain a fixed rate of decrease in the variable being measured, the condensers are shunted by high value resistances which approximate a loss of heat to atmosphere from each condenser. The resist ance across the last condenser is for convenience called the demand. Its value determines the heat flow required from the demand capacity.

The next step in the investigation is to determine the valve size, which is accomplished by setting the range adjuster on the current input unit, namely the resistance 40, so that the full open position of the control valve provides the same rateof rise in the variable being measured as rate of fall when the valve is completely closed. .2.

In the instant investigation, the proper valve range was determined to be 0 to 100 microamperes, corresponding to the lowest and highest controller output pressures. The next step is to check the valve size to insure that a 50% to 75% setting of the control valve maintains the variable being measured at the control point. Any adjustments which may be required may be made by varying the magnitude of resistance 44.

A potentiometer pneumatic controller, as

shown at I in Figs. 1 and 2, may be connected to the process previously described, and adjustments of proportional band (throttling range) and reset rate selected for control of the process. The adjustments are based on minimum area recovery for a supply change. Proportional band is defined as the range of values of the controlled variable which corresponds to the full operating range of the final control element or valve. Reset rate is the number of times per minute that the effect of the proportional action upon the final control element or valve is duplicated by the proportional speed floating action, which, in turn, is defined as a controller action in which there is a continuous linear relation between values of the controlled variable and rate of motion of the final control element.

The supply change may be manually introduced by means of a switch 56, battery 51, slidewire resistance 58 and contact 59 provided in association with the slidewire resistance 5 so that a sudden additional bias voltage is inserted in the sliding contact lead on the control valve slidewire. For example, the supply change may be of such magnitude that when the voltage is introduced, a 25% increase in flow is caused with the actual valve position remaining unchanged. This corresponds to a change in upstream pressure at the control valve, cause a greater flow of control agent, and requires that the controller correct for this change.

The potentiometer controller i may then be calibrated to various spans and the suppression adjusted so that the control point represents a predetermined value: for example, three volts. The controller adjustments may then be redetermined. The following table shows the results obtained for the example being considered, while in Fig. 7 are shown the typical recovery curves for each condition.

- Reset Max Controller Proportional Calibration, Band-8, fif f g q 23:2

Span-volts Percent Scale um volts From a theoretical standpoint, it would be expected that the proportional band would follow a reciprocal law such that as the controller span is reduced by one-half, the proportional band would be doubled. The results of these tests are in approximate agreement as may be seen from the above table. The proportional band is smaller than expected, however, at the smaller scale spans having the greatest amount of suppression.

This discrepancy is capable of explanation as follows. If non-linearities exist in the control system, it is possible that the greater deviation in percent of scale, representing the same deviation in voltage for various spans, may require a smaller proportional band in order to achieve the same stability in the recovery curve. Since the controller is calibrated for linear response of voltage input to air pressure output, it is unlikely that this is the cause of the discrepancy.

A second cause may be the difference in apparent dead zone of the control system with respect to changes in voltage of the process when various controller spans are utilized. For example, a

15 sdea'drzone rof .'04a% of controller-scale withia. six voltsspannwouldnrepresenta11:002e volt-while at a twovoltnspan r'the dead none would represent 19008 1' volt. =Sinoe the" process; possesses-slight itransf-erilag, the decrease in apparentrd'ead zone with; smaller spanscwouldallow a slightly smaller proportional a pand -thus ate leaste partially :accomrting for; the: discrepancy.

Thethird cause; may bezthatcontroller adjust- :ments wereinot set to theEoptim-um values to proiducer-aarecovery, curve-havingthe; smallestpossible, :pe-riodt :wThis may -iurther :account i or the discrepancy-1v JIn fact, therecovery curves of fFig. :7.:indicate :that therperiodz increases slightly as therspan is decreased. :Controller adjustments zwcremade :in each case without changing thereset rate-:from the: previous setting: Its-V315 .found necessary in=every test to decrease the reset: rate in order to iachieve: the desired :stability; Alrthdug'hfthe, resets-.rateshou'ld be unchanged with avarious.:spans-,wtherequireddecrease inreset rate :may "be in ,a-partidue to the method of selecting controller: adjustments. i It is-sworthw'hile to .-note:thatin nsinganytype of analogue-:toruinvestigations in Lautomatic oontrol, it is necessary .to avoid imposing anyilirnitations -on'the operation of theanalogueiunder dynamic conditions. ,If-thecontrol valve is re- ;quired to move to: its I-imit of trave'lyifltherprocess reaches'a potential beyond which it can not go, or if the controller per'i-rshlould attain-J full scale; thewcomplete system is u-nable to follow adequately: its own -laws of operation. Such ac tion in'the:system:correspondinglyiniluences the dynamic operation unless limits are required as in two position c'ontrol.

l rom the ioregoingit is;eviden't that the-electrlcal analogy -method and apparatus herein described for simulating processes lend themselves readily to the selection of varying degrees of process characteristics. It has been shown how such factors as process capacity, transfer lag, dead time, gvalve size controller scale range, and load changes are, set up ontheanalogue.

Various investigations of automatic control, particularly when actual control equipment is employed; emphasize the necessity of; taking into account such factors as measuring lag, controller lag, and ,dead zones in the measuring and. controlling means, as well as in the process. The results of the investigation of, controller span described; emphasize, the fact that theseiactors can be neglectedpnly in exceptional. cases; .3111- piricalmethods of analysis of automatic control mustgenerallyljbe tempered byithe influence of factors "difficult to account for in mathematical methods. ,These factors which are difficult to account for in mathematical methods may be readily takeninto account by means of the apparatus of the'present invention. I

Subject matter disclosed in this application and not claimed herein is disclosed and is be ingrclaimed, in the. copending application" of Donald P; Eckman and William H." Wannamaker, Jrg, Serial No;" 85,l24,1 fi led .March 27, l95i5,"-now-Patent No. 12,470,434, inmy aforementionedPatentNo; 2,453,053, and in my applications Serial ,No. 43,741, new Patent No. 2'; i88-,505,- and; Serial-No.- 431742, both of which were filed on August 11,,'l9 8.

While, indccordance-withtheprovisions of the statutes, -I have-illustrated and described a preferred embodiment of-the-present invention, those skilled in the art willunderstand that oha-ngesmay be made inth-iorm of the appa-- 16 rams-disclosed:without: departing firom the spirit of my invention as set forth in the appended claims, andthat some features of the present in 'vention =may=sometimes be used to advantage without a correspondinguse of other features. I-Iavingnow described-my invention, what I claim as new and desire to secure by Letter Patent, is as -follows:

Litpparatus for simulating a" control system, comprising-a voltage responsive device operative to produce an electrical current varying in amplitude iii-accordance withthemagnitude of, a c011 trol voltage applied thereto; said 'device being characterized in that-the-amplitude of said elec= trical-current'i-s' substantially constant irrespective of the magnitude, over a Wide range, of the loadimpeda'nce through which said current flows, said device-including a, pair of electronicvalves each having an anode, a -ca-thode, and a control gridgsaid electrical current flowing in the anodecathode circuits of saidvalves in series, means to energize said-anode-cathode circuits of said valves to produce-said electric current flow, means toprovide a bias voltage for the control grid cathodeinput circuit of one of said valves includ" ing an adjustable bias resistance connected in the cathode circuit of said one of. said valves, source of potential, and means to provide a biasivoltfor the control grid cathode input circuit of the second ofsaidvalves-including saidrsource of potential and including in the cathode circuit eithesecond of said valves the anode-cathode resistance of said one of said valves andsaid ad- ;iustab-le bias resistance,- an impedance connected in series with saidenergizing means anditheana ode-cathode circuits ofsaid valves, saidi-mpedanceexhibiting electrical characteristics analogous to certain characteristics of, the ,control system being simulated, control means connected between said impedance and saidvoltage-responsive d-evice-and-operative;after thelapse of a time period analogous to, another characteristic oi=-the control-system' bein simulated to apply at least a portion oithe ;vo1 tage drop produced across-said impedance by;the flow of said electrical'current to 'control'said voltage ,responsivedev1ce,= said last mentioned means including a plurality or" condensers, means operative to apply to each of saidcondensers in succession a charge having a magnitude-varying in accordance with the magnitude of said portion of the voltage drop produced across said impedance whereby each of said condenserswil-l have produced thereacross a voltage dependent in magnitude upon the magnitude of the corresponding charge applied theretO,uand means operative to connect to saidvoltage responsive device each of-said; condensers in succession a predetermined time following the application of a charge thereto to apply the voltagesacross'said condensers to control said voltage responsive device, thereby to-control the simulated system represented by the charaflteristics ofsaid impedance, and means connected to said control means and operable to control said voltage-responsive device independently of the control, effected by said portion ofthe voltage drop across said impedance, thereby to effect changes incl/he operating requirements of the simulated control system.

2. Apparatus for simulating a control system, comprising a voltage responsive device having an input circuit and an output c *cu-itand operable to produce an electrical current in said Output circuit varying in'accordancewit-h themagnit-ude of a control'voltageapplied to said input 17' circuit, said device being characterized in that the amplitude of said. electrical current is substantially constant irrespective of the magnitude, over a wide range of variation, of the load impedance through which said current flows, an impedance exhibiting electrical characteristics analogous to certain characteristics of the control system being simulatedrmeans to pass said electrical current through said impedance to produce a voltage drop, control means to derive from said voltage drop a control voltage of corresponding magnitude, means connected between said impedance and the input circuit of said voltage responsive device and operative to apply said control voltage to control said voltage responsive device, thereby to control the. simulated system represented by the characteristics of said impedance, the last mentioned means including a plu-- rality of condensers having a common terminal, a multiple contact switch having an operating shaft and having'two sections each provided with a: switch arm. secured to said shaft and with an associated plurality of contact points, each of which is connected to an individual one of the other terminals of said condensers, conductor meansconnected between said control'means and said lastmentioned means and operative to apply saidcontrol voltage between one of said switch arms, and said common terminal of said condensers, means connecting the other of said switch arms and said common terminal of said condensers to the input circuit of said voltageresponsive device, said switch arms being so arranged on said shaft with respect to each other and the contact points on their associated switch sections that said one switch. arm applies said control voltage to eachof said condensers before said second switch arm connects each of said condensers to the input circuit of said voltage responsive device, and means to cause said switch arms to successively engage each ofthe contact points on their associated switch sections, comprising a, plurality of electronic valves, each of said valves having an inputcircuit andan output circuit, connections to a source of power for supplying alternating current to said electronic valves, conductor means for so connecting said electronic valvesto thesource of power that one of,

said valves is operative on only one half of each.

cycle of saidalternating current whileanother of said valves is operative on only the alternate half cycles of said alternating current, adjustable bias means connected in the input circuit of said one valve, a condenser connected in the output circuit of said one valve, the input circuit of said other valve being connected across the last mentioned condenser, a relay having anoperating windingv connected in the output circuit of said other valve, a movable member actuable from one position to another position and operative, when actuated, tov advance each of said switch arms from one contact point. on itsassociated switch section to. an adjacent contact point thereon, a second relay controlled by the first mentioned relay andioperative to actuate said movable member, and switch means operated'in' unison with said movable member to shunt said last mentioned condenser in the output circuit of said one valve when said movable member is in one of said positions, and means connected to said control means and operable to control said voltage responsive device independently of the control effected by said voltag drop across said impedance, therebyto effect changes in the operating requirements of the simulated control system.

3. Apparatus for simulating a control system, comprising means to produce an electrical current having a substantially constant amplitude irrespective of the magnitude, over a wide range of variation, of the load impedance through which said current flows, said means including a pair of electronic valves each having aninput circuit and having an output circuit through which said current flows, means to supply energizing voltage to the output circuits of said valves in series, self bias means connected in the input circuit of one of said valves, and bias means for the second of said valves including the output circuit of said one of said valves and said self bias means, an impedance connected in series with the output circuits-of said valves, said impedance exhibiting electrical characteristics analogous to certain characteristics of the control system being simulated, means connected to said impedance and operative to. measure the voltage drop produced across at least a portion of said impedance by the fiow of said current, control means connected between the last mentioned means and the first mentionedmeans and operative to derive from said voltage drop a control voltage of corresponding magnitude, said control means including means connected to said input circuit or" said one of said valves, whereby said control voltage controls said current flow to control the simulated system represented by the characteristics 01' said impedance, and means connected to said control means and operable to control said first mentioned means independently of the control efiected by said voltage drop, thereby to effect changes in the operating requirements of the simulated control system.

4. Apparatus for simulating a control system, comprising means to produce an electrical current having a substantially constant amplitude irrespective of themagnitude, over a wide range of variation, of the load impedance through which said current flows, said means including a pair of electronic valves each having an anode, a cathode, and a control grid, said electrical current flowing inthe anode-cathode circuits of said valves, means to energize in series the anode-cathode circuits of said valves to produce said electric current flow, means to provide a bias voltage for the control grid cathode input circuit of one of said valves including an adjustable bias resistance connected in the cathode circuit of said one of said valves, a source of potential, and means to provide a bias voltage for the control grid cathode input circuit of the second of said valves including said source of potential and including, in the cathode circuit of the second of said valves the anode-cathode resist.- ance of said one of said valves and said adjustable bias resistance, an impedance connected in series with said energizing means and the anode-cathode circuits of each of said valves, said impedance exhibiting electrical characteristics analogous to certain characteristics of the control system beingv simulated, means connected to said impedance and operative to measure the voltage drop produced across at least a portion of said impedance by the flow of said current, control means connected between the last mentioned means and the first mentioned means and operative to derive from said voltage drop a control voltage of corresponding magnitude, said control means including means connected to said input circuit of said one of said valves, whereby said control voltage controls said current flow to control the simulated system represented by the characteristics of said impedance, and means connected to said control comprising means operative to produce an electrical current of adjustable amplitude but having a substantially constant amplitude irrespective of the magnitude, over a wide range of variation, of the load impedance through which said current flows, said means including a pair of electronic valves each having an anode, a cathode, and a control grid, said electrical current flowing in the anode-cathode circuits of said valves, means to energize in series the anode-cathode circuits of said valves to produce said electric current flow, means to provide a bias voltage for the control grid cathode input circuit of one of said valves including an adjustable bias resistance connected in the cathode circuit of said one of said valves, 2. source or potential, and means to provide a bias voltage for the control grid cathode input circuit of the second of said valves including said source of'poten-tial and including in the cathode circuit of the second of said valves the anode-cathode resistance or" said one of said valves and said adjustable bias resistance, an impedance connected in series with said energizing means and the anode-cathcde circuits of said valves whereby said electrical current iiows through said impedance, said impedance exhibiting electrical characteristics analogous to certain characteristics of the control system being simulated, control means connected between said impedance and the first mentioned means and responsive to the magnitude of the voltage produced across at least a portion of said impedance by said electrical current flow to apply a potential of corresponding magnitude between the control electrode and cathode of said one valve to thereby vary the amplitude of said electrical current flow, thereby to control the simulated system represented by the characteristics of said impedance, and means connected to said control means and operable to control said first mentioned means independently of the control eiiected by said voltage produced across said impedance, thereby to efieot changes in the operating requirements of the simulated control system.

6. Apparatus for simulating a control system, comprising a voltage responsive device operative to produce an electrical current varying in amplitude in accordance with the magnitude of a control voltage applied thereto, said device being characterized in that the amplitude of said electrical current is substantially constant irrespective of the load impedance through which said current flows, an impedance exhibiting electrical characteristics analogous to certain characteristics of the control system being simulated, means to pass said current through said impedance to produce a voltage dropthereacross, control means connected between said impedance and said voltage responsive device and operative to vary the amplitude of said current, including a plurality of condensers and means operative to apply to each of said condensers in succession a charge having a magnitude varying in accordance with the magnitude of at least a portion of said voltage drop produced across said impedance, whereby each of said condensers will have produced thereacross a voltage dependent in magnitude upon the magnitude of the corresponding charge applied thereto, and operative thereafter to connect to said voltage responsive device each of said coridenser-s in succession to produce a control voltage for controlling said voltage responsive device, thereby to control the simulated system represented by the characteristics of said impedance, and means connected to said control means and operable to control said voltage responsive device independently or" the control effected by said portion of the voltage drop across said impedance, thereby to effect changes in the operating requirements of the simulated control system.

'7. Apparatus for simulating a control system;

comprising a voltage responsive device operative to produce an electrical current varying in am: plitude in accordance with the magnitude of a control voltage applied thereto, said device being characterized in that the amplitude of said electrical current is substantially constant irrespective of current flows, an impedance exhibiting electrical characteristics analogous to certain characteristics of the control system being simulated, means to pass said current through said impedance to produce a voltage drop thereacross, means connected between said impedance and said voltage responsive device and operative to vary the amplitude of said current, including a plurality of condensers, switch means to apply to each of said condensers in succession a charge having a magnitude varyingin accordance with the magnitude of at least a portion of said voltage drop produced across said impedance, whereby each of said condensers will have produced thereacross a voltage dependent inmagnitude upon the magnitude of the corresponding charge applied thereto, means to connect to said voltage responsive device each of said condensers in succession a predetermined time following the application of a charge thereto to derive a control voltage, means to apply said control voltage to control said voltage responsive device,v means controlling said switch means to vary the length of said time, thereby to control the simulated system represented by the characteristics of said impedance, and means connected to said control means and. operable to control said voltage responsive device independently of the control effected by said portion of the voltage drop across said impedance, thereby to effect changes in the operating requirements of the simulated control system.

8. Apparatus for simulating a control system, comprising a voltage responsive device having an input circuit and an output circuit and operable to produce an electrical current in said output circuit varying in accordance with the magnitude of a control voltage applied to said input circuit,

said device being characterized in that the amplitude of said electrical current is substantially constant irrespective of the magnitude, over a wide range of variation, of the load impedance through which said current flows, an impedance exhibiting electrical characteristics analogous to certain characteristics of the control system being simulated, means to pass said electrical current through said impedance to produce a voltage drop, control means connected between said impedance and the input circuit of said voltage responsive device and operative'to derive from said voltage drop a control'voltage of corresponding magnitude, means to apply said control voltage to control said voltage responsive device, thereby to control the simulated system representedby the characteristics of Said impedance, the last' mentioned means including a plurality of QOI1- b e load impedance through which said control densers having a common terminal, a multiple contact switch having an operating shaft and having two sections each provided with a switch arm secured to said shaft and with an associated plurality of contact points, each of which is connected to an individual one of the other terminals of said condensers, conductor means connected between said control means and said last mentioned means and operative to apply said control voltage between one of said switch arms and said common terminal of said condensers, means connecting the other of said switch arms and said common terminal of said condensers to the input circuit of said voltage responsive device, drive means to cause each of said switch arms to successively engage each of the contact points on its associated switch section, said switch arms being so arranged on said shaft with respect to each other and the contact points on their associated switch sections that said one switch arm applies said control voltage to each of said condensers before said second switch arm connects each of said condensers to the input circuit of said voltage responsive device, and means connected to said control means and operable to control said voltage responsive device independently of the con trol effected by said voltage drop across said im pedance, thereby to effect changes in the operating requirements of the simulated control system.

9. Apparatus for simulating a control system, comprising a device operative to produce an electrical current having a substantially constant amplitude irrespective of the load impedance through which said current flows, an impedance exhibiting electrical characteristics analogous to certain characteristics of the control system being simulated, means to pass said current through said impedance to produce a voltage drop across said impedance, means connected to said impedance and operative to measure at least a portion of said voltage drop including a vacuum tube voltmeter and an exhibiting device controlled by said vacuum tube voltmeter, control means connected between the last mentioned means and the first mentioned device and operative to derive from said voltage drop a control voltage of corresponding magnitude and to apply said control voltage to control said current flow, thereby to control the simulated system represented by the characteristics of said impedance, and means connected to said control means and operable to control said first mentioned device independently of the control eifected by said voltage drop, thereby to eifect changes in the operating requirements of the simulated control system.

10. Apparatus for simulating a control system, comprising a device operative to produce an electrical current having a substantially constant amplitude irrespective of the load impedance through which said current flows, an impedance exhibiting electrical characteristics analogous to certain characteristics of the control system being simulated, means to pass said current through said impedance to produce a voltage drop across said impedance, said voltage drop having a magnitude jointly determined by the amplitude of said current and by the characteristics of said I impedance, a measuring instrument having an input circuit, the impedance of which is substantially smaller than the impedance across which said voltage drop is produced, and having an output connection, an impedance matching vacuum tube voltmeter having an input circuit connected to said impedance and controlled in accordance with the magnitude of said voltage drop and having an output circuit connected to the input circuit of said measuring instrument, control means connected between said output connection of said measuring instrument and said device and operative to control the amplitude of said current in accordance with the magnitude of said voltage drop, thereby to control the simulated system represented by the characteristics of said impedance, and means connected to said control means and operable to control the amplitude of said current independently of the control effected by said voltage drop, thereby to effect changes in the operating requirements of the simulated control system.

11. Apparatus for simulating a control system, comprising a device operative to produce an electrical current having a substantially constant amplitude irrespective of the load impedance through which said current flows, an impedance exhibiting electrical characteristics analogous to certain characteristics of the control system being simulated, means to pass said current through said impedance to produce a voltage drop across said impedance, means to measur at least a portion of said voltage drop including an electronic valve having an input circuit, connected to said impedance and to which said voltage drop is applied, and having an output circuit, said output circuit including in series a voltage supply source and an adjustable resistance across which there appears a potential drop, a second adjustable resistance connected to said voltage supply source, whereby there appears across said second adjustable resistance a voltage drop having a magnitude dependent on the adjustment of said second adjustable resistance, and exhibiting voltage responsive device having input terminals, and means to apply the dilTerence of said potential drops across said resistances between the input terminals of said exhibiting device, control means connected between said measuring means and the first mentioned device and operative to derive from said voltage drop a control voltage of corresponding magnitude and to apply said control voltage to control said current flow, thereby to control the simulated system represented by the characteristics of said impedance, and means connected to said control means and operable to control said current flow independently of the control efiected by said voltage drop, thereby to effect changes in the operating requirements of the simulated control system.

WILLIAM H. WANNAMAKER, JR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Certificate of Correction Patent No. 2,519,615 August 22, 1950 WILLIAM H. WANNAMAKER, J n.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows:

Column 17, lines 12 to 14:, strike out the Words connected between said impedance and the input circuit of said voltage responsive device and operative and insert same after means in line 10, same column;

and that the said Letters Patent should be read as corrected above, so that the same may conform to the record of the casein the Patent Oflice.

Signed and sealed this 19th day of June, A. D. 1951.

THOMAS F. MURPHY,

Assistant Commissioner of Patents.

Certificate of Correction Patent No. 2,519,615

WILLIAM H. WANNAMAKER, JR.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows:

4 Column 17 lines 12 to 14, strike out the Words connected between said impedance and the input circuit of said voltage responsive device and operative and insert same after means in line 10, same column;

and that the said Letters Patent should be read as corrected above, so that the same may conform to the record of the case in the Patent Oflice.

Signed and sealed this 19th day of June, A. D. 1951.

August 22, 1950 THOMAS F. MURPHY,

Assistant Uommz'ssioner of Patents. 

